In microelectronics device fabrication there is a need to fill narrow trenches having aspect ratios (AR) greater than 10:1 with no voiding for many applications. One application is for shallow trench isolation (STI). For this application, the film needs to be of high quality throughout the trench (having, for example, a wet etch rate ratio less than two) with very low leakage. One method that has had past success is flowable CVD. In this method, oligomers are carefully formed in the gas phase which condense on the surface and then “flow” into the trenches. The as-deposited film is of very poor quality and requires processing steps such as steam anneals and UV-cures.
As the dimensions of the structures decrease and the aspect ratios increase post curing methods of the as deposited flowable films become difficult. Resulting in films with varying composition throughout the filled trench.
Amorphous silicon has been widely used in semiconductor fabrication processes as a sacrificial layer since it can provide good etch selectivity with respect to other films (e.g., silicon oxide, amorphous carbon, etc.). With decreasing critical dimensions (CD) in semiconductor fabrication, filling high aspect ratio gaps becomes increasingly sensitive for advanced wafer fabrication. Current metal replacement gate processes involve a furnace poly-silicon or amorphous silicon dummy gate. A seam forms in the middle of the Si dummy gate due to the nature of process. This seam may be opened up during the post process and cause structure failure.
Conventional plasma-enhanced chemical vapor deposition (PECVD) of amorphous silicon (a-Si) forms a “mushroom shape” film on top of the narrow trenches. This is due to the inability of the plasma to penetrate into the deep trenches. The results in pinching-off the narrow trench from the top; forming a void at the bottom of the trench.
Conventional thermal CVD/furnace processes can grow a-Si via thermal decomposition of a silicon precursor (e.g., silane). However, due to the inadequate precursor supply or presence of decomposition byproduct, the deposition rate is higher on top of trenches comparing with it at the bottom. A narrow seam or void can be observed in the trench.
Therefore, there is a need for methods for gapfill in high aspect ratio structures that can provide seam-free film growth.